We are investigating aspects of cell-mediated cytotoxicity in humans: (1)\analyzing the phenomenon of interferon induction in mixed cultures of lymphocytes and virus-infected or tumor-derived cells; (2)\investigating the enhancing effect of interferon and interferon inducers on natural killer (NK) cells; (3)\studying the mechanism of interferon-induced resistance to lysis of target cells; (4)\identifying cytotoxic cell types generated during an immune response; (5)\analyzing the specificity and the genetic control of cytotoxic T cells against virus-infected and transformed cells; and (6) biosynthesizing, by somatic cell hybridization, monoclonal antibodies against lymphocyte surface antigens able to identify lymphocyte subpopulations. Recent studies are mostly focused on NK cells and, in particular, on their surface markers, including Fc-receptor, and on their activation and growth regulation by interferon, interleukins, and other factors or stimuli. Interleukin 2 (IL-2) induce a rapid enhancement of NK activity, as well as production of immune interferon, IL-2, in the presence of feeder cells, also induce proliferation of the NK cell subset. Proliferating NK cells express a series of activation antigens, similar to those expressed by proliferating T cells. NK cells suppress certain types of hematopoietic colonies: we have found that NK cells in contact with sensitive target cell lines or with normal bone marrow cells produce a colony suppressing activity (NK-CSA), probably identical to the NK-derived cytotoxic factor, that suppresses the same type of colonies inhibited by NK cells. NK-CSA production is induced by the most immature bone marrow cells, and it is also absorbed and effective on the same immature subset. (SR)